The output of the switching power supply is not a pure DC voltage, but there are some AC components in it, which is caused by ripple and noise. Ripple is the fluctuation of the output DC voltage, which is related to the switching action of the switching power supply. During each turn-on and turn-off process, electric energy is "pumped" from the input end to the output end, forming a charging and discharging process, resulting in fluctuations in the output voltage, and the fluctuation frequency is the same as the switching frequency. Ripple voltage is the peak-to-peak value between the peak and trough of the ripple, and its size is related to the capacity and quality of the input capacitor and output capacitor of the switching power supply.

The noise generated by the switching power supply itself is a high-frequency pulse train, which is caused by the sharp pulses that occur at the moment when the switch is turned on and off, and is also called switching noise. The frequency of the noise burst is much higher than the switching frequency, and the noise voltage is its peak-to-peak value. The amplitude of the noise voltage is largely related to the topology of the switching power supply, the parasitic state in the circuit and the design of the PCB.

Ripple and noise waveforms can be seen with an oscilloscope, as shown in Figure 1. The frequency of the ripple is the same as that of the switching tube, and the frequency of the noise is twice that of the switching tube. The sum of the peak-to-peak value of the ripple voltage and the peak-to-peak value of the noise voltage is the ripple and noise voltage, and its unit is mVp-p.

A block diagram of a device for measuring ripple and noise with an oscilloscope is shown in Figure 2. It consists of the tested switching power supply, load, oscilloscope and measurement connection. In some measuring devices, components such as inductance or capacitance and resistance are also welded.